Circuits for providing a reference voltage (hereinafter reference voltage circuits) are utilized in a variety of applications. It is also very important in many applications that the reference voltage (V.sub.REF) be independent of the supply voltage (V.sub.CC) and environmental conditions.
Such circuitry in need of voltage independence can be found in a myriad of specific and general electronic applications; especially in those circuits of many products wherein it is an absolute requirement to have a very stable voltage source which in turn can be used internally within the circuits power other sub-circuits.
For example, in the art of flash memory one may want to program (insert information) into one of the memory cells. In order to achieve this, one needs to construct an internal pump which acts to internally establish a particular program voltage level. Likewise, in the instance when you need to erase a particular memory cell a pump is also needed. This pumping action of a voltage has the inherent problem of varying substantially according to the V.sub.CC and the operating temperature of the circuit due to the varying temperature coefficients of the circuit components. These variations effect the circuits operating frequency characteristics. The requirement for the pump output, in order to be able to perform the erase on some memory cells, needs to stay relatively constant in order to have the same characteristics as the programming such that the erase operation is complete and effective.
To achieve this stable program/erase voltage, a very stable reference voltage V.sub.REF is required which is held to a constant level. For instance, if the reference voltage is at a level of, for example, two (2) volts and the erase/program requirements are such that only nine (9) volts are required then the pump must be sourcing a steady 9 volt supply. Otherwise depending on other parameters such as temperature and process variations, if any, the output of the pump may simply vary too much. In many cases, five (5) volt components need to be pumped to maintain specific predetermined signal margin levels which do not fluctuate beyond selected parameters.
Furthermore, there are other circuits, such as the power-on-reset circuit, which is a circuit having a characteristic such that it can effectively lockout a low V.sub.CC, can make use of a very stable V.sub.REF also. In this instance, when the V.sub.CC or the supply voltage is too low it may affect the circuit operation. In these particular circuits, a stable V.sub.REF has a particular usage. Similarly, there are many other applications for a very stable V.sub.REF also such as in a DRAM design.
More particularly, in this art specific problems exist with establishing and maintaining proper bias conditions within the circuits which are independent of varying chip operating temperatures and differing supply voltage variations within the chip's many individual circuit components. In addition, additional problems are inherent in the manufacture of the chip wafer which may add process variations to the surface on which these many millions of components will be placed.
Because of the increasing complexity of the circuitry contained on a single integrated circuit (IC) chip, minimization of power dissipation is also of particular importance from a packaging perspective. Fluctuations in bias current with temperature, supply voltage, and process variations can often result in power dissipation problems which may impact the design objectives of the circuit. As such, supply-independent bias circuitry is particularly important in order to avoid the injection into the signal path of the various components of the IC-circuit spurious and deleterious high-frequency signals that may quite often be present on the power-supply lines. Critical in this regard is the ability to achieve a degree of supply voltage independence between the many circuits on the chip and the supply voltage source(s) made available to the various chip components such that the biasing circuits are referenced to some voltage potential other than that of the supply voltage.
The prior art, MOS (metal oxide semiconductor), includes the use of a threshold voltage, the use of the difference between the threshold voltages of dissimilar devices, the use of the base-emitter voltage of some parasitic bipolar transistor device, the use of the thermal voltage, and the use of the band-gap voltage. Furthermore, the use of self biasing in these circuits may dramatically improve supply independence. However, these approaches often require the implementation of a stand alone start-up circuit which, when activated upon power-on, helps prevent the circuit from reaching equilibrium in some state other than that state desired by the circuit designer to be optimal for tolerated circuit function.
In the instance wherein voltage supply independence is achieved by use of threshold-referenced biasing, in a V.sub.t -referenced self-biased threshold-referenced circuit feedback is produced by transistors which forces the same current to flow in another transistor as that which flows through resistor. Here, temperature and supply dependence remains basically the same.
Another important aspect of the performance of this type of self-biased circuit is stability at the desired operating point. However, self-biased circuits often are very dependent on positive feedback. Thus, to maintain a fair degree of stability, it must be predetermined that the feedback loop gain is actually less than unity at the desired operating point by breaking the loop, injecting a signal therein, and ascertaining that the loop gain is less than unity.
Another important aspect of the performance of bias circuits is the degree of supply independence that can be achieved in the circuit's bias currents and voltage levels. In this particular instance, the channel-length modulation in the transistors may cause variations in the levels of bias current which to a fair degree may be minimized by the use of cascode current sources.
It is very important to note that these type voltage threshold-referenced bias circuits have inherent problems, in that, in most MOS processes the threshold voltage is not particularly well controlled, (a range of threshold voltages typically would be from 0.5 to 0.8 V). Another problem, albeit more tangentially, is that in these type circuits often the threshold voltage of an n-channel MOS transistor displays a relative negative temperature coefficient whereas diffused resistors quite often display a substantially positive temperature coefficient to the effect that the output current has a large negative temperature coefficient.
An alternative approach to threshold referencing is the use of the difference between the threshold voltages of two semiconductor devices having the same polarity but which also have differing channel implants such that the temperature coefficients of the two threshold voltages cancel to first order. However, one disadvantage of this type of implementation of a voltage reference is the large initial tolerance in the output voltage value because of the relatively high tolerance on the threshold voltages which must be dealt with. In these instances, the absolute output voltage can be effectively adjusted by trimming.
In the instance wherein voltage supply independence is achieved by use of Base-Emitter-referenced biasing (or V.sub.BE,-referencing), a typical V.sub.BE -referenced bias circuit includes a pnp-transistor as the parasitic device that is inherent in p-substrate CMOS technologies. Alternatively, a corresponding circuit utilizing npn-transistors can be implemented in n-substrate CMOS technologies. It should be noted that this particular biasing method is not available in NMOS technology because of the lack of a diode or transistor. This configuration of V.sub.BE -referenced biasing has the advantage that the V.sub.BE of a bipolar transistor is a relatively well-controlled component characteristic typically having a variation of 5 percent of its value as a result of inherent processing variations.
However, the disadvantages are such that the V.sub.BE displays a negative temperature coefficient which when coupled with the strong positive temperature coefficient of the diffused and poly-silicon type resistors therein may result in a relatively highly negative temperature coefficient in the overall bias current of the circuit. Also and in the alternative as with the threshold-referenced type circuit, the variation of reference current with spurious power-supply fluctuations can be minimized by the use of cascode or Wilson current sources.
In the instance wherein voltage supply independence is achieved by use of Thermal Voltage (V.sub.t)-referenced current sources, a V.sub.t -referenced self-biased reference circuit wherein two transistors having areas that differ by a set factored amount and a feedback loop therein allows these two transistors to operate at the same bias current level such that the difference between the two Base-Emitter voltages (V.sub.BE) is across resistor R. The primary advantage of this type circuit implementation is that the thermal voltage V.sub.t has a positive temperature coefficient and, when taken in conjunction with the positive temperature coefficient of the resistor, a relatively temperature-independent output current results.
However, it should be understood that in this type circuit, small differences in the gate-source voltages of components of such a circuit may result in relatively large fluctuations in the resulting output current level due, primarily to the fact that the voltage differential established across the resistor is relatively small (on the order of about 100 mV) caused from component mismatches. This may also result from channel-length modulation activity in the transistors due to their differing drain voltage potentials. Prior art implementations of this type circuit typically will often utilize relatively large device for the components in order to minimize gate-source voltage offsets therein and will often utilize cascode or Wilson current sources in order to minimize channel-length modulation effects.
All of the above-identified reference voltage circuits are process and temperature sensitive. Therefore some additional circuitry is necessary to counteract that temperature and process sensitivity. This additional circuitry increases the cost and complexity of the reference voltage circuit. What is needed is a voltage reference circuit that is supply voltage independent, while at the same time does not require the additional circuitry necessary for adjustment for the proper operation of prior independent reference voltage circuit. The present invention addresses such a need.